Jaguar Repair: low power and backfire up hills, compression gauge, vacuum advance

Question
I have a 1969 XKE 4.2 series 2.  When going up steeper hills the car loses power and backfires.  I have been reading different sites and suspect it may be the vacuum advance mechanism affecting the ignition timing.  Do you have any thoughts?  Could is also be a carb problem, or something else?  What would be the simplest checks first?  I am just learning the car so I don't know a lot about diagnosis/repairs.  The car came from my father in law.  Thank you.

Answer
Hi Ethan
Your symptoms only narrow the problem down to a thousand things so you must test to get closer to the problem. It does require a few tools but they are not expensive. You will need a plug wrench, timing light, 12v test light, a compression gauge and normal hand tools. It is not necessary to purchase professional tools unless you plan to work on cars for a living.

First and most important is a compression test. (throttle open) You want to see 135 to 165 PSI on all cylinders with little difference between cylinders.

After that is established as correct you can proceed to checking fire and fuel. The fire (ignition) can be checked in a few minutes at any shop with a scope. It should not cost more than about #35.00 to run a test on the ignition. A shop with a scope does not need to know anything about a Jaguar to run the test and it is more sure and much faster than you can do the tests necessary to rule out ignition as a fault. It would also help if you would put in a fresh set of spark plugs before you run the test. This clears up a lot on their screen when testing. I have been a Jaguar mechanic for over 40 years and I still take my own XJ-6 to a shop to test the ignition system just because the test is cheep and tells everything about the ignition system.

If the compression test and the ignition test does not show up the problem you then can look at fuel. First test is fuel pressure at the carburetors. You want to see at least 2 to 3 PSI at the carburetors when running and it is a good test to use a long hose an put the gauge under a wiper arm so you can see the fuel pressure when climbing that hill that makes it loose power and back fire. If it starts to loose power and still has good pressure you then know for sure it is not fuel supply to the carburetors that is the problem.

If after all this you still have the problem you need to go into the Stromberg carburetors and this is a problem that you should obtain a service manual to do. It is not difficult but you need the procedure laid out in front of you so you do not get tripped up. If you have any problems when doing this you should contact me in this program and I will help.

Howard